Deadly H5N1 may be brewing in cats

Bird flu hasn’t gone away. The discovery, announced last week, that the H5N1 bird flu virus is widespread in cats in locations across Indonesia has refocused attention on the danger that the deadly virus could be mutating into a form that can infect humans far more easily. In the first survey of its kind, an Indonesian scientist has found that in areas where there have been outbreaks of H5N1 in poultry and humans, 1 in 5 cats have been infected with the virus, and survived. This suggests that as outbreaks continue to flare across Asia and Africa, H5N1 will have vastly more opportunities to adapt to mammals than had been supposed. Chairul Anwar Nidom of Airlangga University in Surabaya, Indonesia, told journalists last week that he had taken blood samples from 500 stray cats near poultry markets in four areas of Java, including the capital, Jakarta, and one area in Sumatra, all of which have recently had outbreaks of H5N1 in poultry and people.

Of these cats, 20 per cent carried antibodies to H5N1. This does not mean that they were still carrying the virus, only that they had been infected – probably through eating birds that had H5N1. Many other cats that were infected are likely to have died from the resulting illness, so many more than 20 per cent of the original cat populations may have acquired H5N1. This is a much higher rate of infection than has been found in surveys of apparently healthy birds in Asia. “I am quite taken aback by the results,” says Nidom, who also found the virus in Indonesian pigs in 2005. He plans further tests of the samples at the University of Tokyo in February. Amin Soebandrio, head of medical sciences at the Indonesian ministry for research and technology, confirmed the report. He says that the infection has also been found in dogs and cats on the Indonesian island of Bali, which has also had outbreaks of H5N1. The new findings follow reports that unusually large numbers of dead cats have been found near many outbreaks of H5N1. “Javanese farmers even have a word for the cat disease,” says Albert Osterhaus of Erasmus University in Rotterdam, the Netherlands. It was Osterhaus’s lab which in 2004 found that cats can catch the H5N1 virus. Like humans, some cats die, and some recover. But unlike humans, infected cats shed large amounts of the virus and pass it to each other.

Infected cats may not directly increase the danger of people catching the virus, as humans seem to catch the current strain only with difficulty even from birds, which they kill, pluck and eat. The main worry, says Osterhaus, is that as the virus replicates in cats it will further adapt to mammals and acquire the ability to spread more efficiently to people and from person to person, unleashing a human pandemic. Nidom’s findings are the first to indicate what proportion of cats can become infected by H5N1. No cats have been tested in Hong Kong or China. In Bangkok, Thailand, all the cats in one household are known to have died of H5N1 in 2004. Tigers and leopards in Thai zoos also died, while last year two cats near an outbreak in poultry and people in Iraq were confirmed to have died of H5N1, as were three German cats that ate wild birds. In Austria cats were infected but remained healthy (New Scientist, 18 March 2006, p 6).

Though Osterhaus says Nidom’s figures must be confirmed, he says they aren’t surprising, and is even encouraged that they aren’t worse. A higher percentage of infected predators than prey makes sense, as each predator eats many prey animals. “At least that percentage shows the virus has not completely adapted to cats – yet,” Osterhaus says. If it had, all cats in a stricken area should be infected, as with ordinary flu in humans. Osterhaus emphasises that the cat infections still pose a potential threat. “We know the 1918 pandemic was a bird flu virus that adapted to mammals in some intermediate mammalian host, possibly pigs,” he says. “Maybe for H5N1 the intermediate host is cats.” If similar percentages of cats are infected at every outbreak location, there must have been many thousands of cat infections since the virus emerged, compared to 267 confirmed cases in humans. Every sick cat is a chance for the virus to adapt, and with renewed outbreaks this year in birds, people or both in China, Indonesia, Japan, South Korea, Hong Kong, Vietnam, Thailand, Egypt and Nigeria, it is getting plenty of such chances.

“In the 1918 flu pandemic there was some intermediate mammalian host. For H5N1 that host could be cats ”Killing cats won’t solve the problem, Osterhaus warns. Like shooting wild birds, it is unlikely to have much impact and could send infected animals elsewhere. It would also lead to a population explosion of disease-carrying rodents, which the cats normally keep in check. “Killing cats won’t solve the problem and would lead to a population explosion of disease-carrying rodents” “Cats must just be kept from eating sick chickens,” Osterhaus says, though this will be a tall order in open-air markets across Asia and Africa, which are typically swarming with hungry cats. In Jakarta this week, officials are slaughtering thousands of banned backyard poultry – then handing them back for their owners to eat. Some of the birds could well be infected despite appearing healthy. It is hard to imagine the local cats not getting their share.